This invention relates generally to the field of steam heat kiln systems and more specifically to the method and apparatus for detection of waterlogging in steam heating coils.
Steam heat coil systems are used in lumber drying operations in part due to the ability of steam to release latent heat efficiently and relatively uniformly across an array of horizontal fin pipe, known as coils, which are spaced apart in a parallel pattern and connected by header pipes. One section of coils connected by a header pipe on either side is herein referred to as a steam heating coil panel.
Steam is pressured through this coil arrangement in order to provide uniform heat distribution across the coil grid and consequently, to impart a relatively uniform heat output, both vertically and horizontally, to the lumber being dried. If the heat released from the steam heating coil panel is not relatively uniform and varies by more than a few degrees, either vertically or horizontally across the plane of the coil panel, the lumber will dry significantly less uniformly, resulting in a higher percentage of damaged wood products.
Condensed steam, called condensate, is typically removed from the steam heating coils via a steam trap and returned to a boiler for conversion back to steam. When condensate is not effectively removed from steam heating coils and is allowed to build up, it creates a condition called waterlogging. Condensation build-up resulting in waterlogging can be caused by a variety of reasons including back pressure on the steam trap at the end of the coil grid, inadequately sized condensate piping, undersized and/or plugged steam traps, or insufficient steam pressure.
Waterlogging is known to cause disparities in heat distribution of the kiln by preventing an adequate amount of steam from releasing latent heat in the area of the pipes experiencing the condensate build-up. In addition, higher energy costs are often incurred when waterlogging is present near the system's resistance temperature detector (RTD) because the system toggles an increase of steam temperature to the kiln coils to compensate for the decrease in temperature at the RTD due to waterlogging at that point.
Due in part to the harsh environment inherent in this type of operation and lengthy operational cycle time, an effective method of detecting the occurrence of condensate build-up in the steam heating coils of a closed system throughout a significant portion of the operational cycle of a kiln has not been developed before. This has resulted in many kiln operations producing a higher percentage of damaged product and often at an increased cost in energy. In addition, condensate build up has a corrosive effect on the coil pipes resulting in permanent damage, increased chemical treatment costs, and premature aging of the system. Detecting waterlogging in kiln coils is often necessary before corrective measures can be put in place and further, any corrective measures cannot be determined to be adequate without an effective means of subsequent evaluation.
While it is known in the industry that build-up of condensate in a steam heating system reduces efficiency and increases operating costs, common practices to identify the occurrence of condensate build-up and waterlogging generally focus on steam traps in the system including visual inspections of steam trap discharge, temperature and pressure measurements taken on steam traps, and acoustic signals from steam traps.
Both U.S. Patent Application Publication No. U.S. 2006/0122808 A1 and U.S. Patent Application Publication No. U.S. 2007/0073495 A1 disclose methods for analyzing steam trap data by the placement of monitoring devices on or near a steam trap and the subsequent capture and analysis of the data to indicate steam trap operating conditions.
U.S. Patent Application Publication No. U.S. 2001/0007093 A1 proposes an evaluating system that uses steam trap surface temperatures along with measurements of the level of vibrations at the steam trap to determine the extent of steam trap leakage.
While measurements of steam trap conditions can be analyzed for reasonable indications of steam leakage through a steam trap or indicate the potential for condensate build-up in the system, steam trap analysis alone does not provide adequate indications of the extent of condensate build-up within a system, the impact of waterlogging on the efficiency of the system, or the corrosive effects of waterlogging within the specific coils where it is taking place. These methods only provide data specific to the location of the steam trap and the efficiency of the steam trap being measured and do not address the efficiency of the heat distribution across the entire system, of which the steam trap is only one component.
In a steam heat drying kiln operation such as those used for drying lumber, it is important to understand the extent to which condensate build-up and waterlogging may be occurring across the entire system of heating coils and throughout the length of the drying cycle which can operate continuously for more than 30 hours. Without knowledge of the degree of waterlogging taking place, and at what point in time it is likely to occur within a system, it is difficult to gauge the efficiency of the system and consequently, the economic impact of the performance of the system or make an economic evaluation of the value of improving it. Consequently, evaluation of steam trap efficiency alone will not provide the needed indication of the system performance as a whole or the effectiveness of attempted corrections to the system, especially if the root cause of the waterlogging condition is due to something other than the steam trap operation.
Consequently, a need exists to be able to identify condensate build-up at varying vertical distances of the heating coils throughout the length of the drying cycle. U.S. Patent Application Publication No. U.S. 2006/0070438 A1 proposes a method of determining a density-compensated liquid level in a vessel containing a mixture of liquid and vapors by the use of pressure and temperature sensors located on or near the containment vessel in combination with calculations utilizing the specific gravities of the contained liquids. U.S. Patent Application Publication No. U.S. 2004/0181349 A1 describes a software-based water level monitoring and control system for use with conventional steam boilers through the use of level sensors in combination with a computer monitoring system. Both of these processes describe methods of identifying liquid levels within a specific container, however and do not address the challenges of measuring condensate build-up within heating coils containing steam moving through them continuously. Further, both processes require the use of measurement devices that must be inserted into the body of the vessel, which requires extensive modifications for installation in a system, nor do they provide for storing, displaying, and analysis of measurements of conditions over the length of a typical operational cycle in order to identify the degree to which waterlogging may be occurring.
There is therefore a need in the art for a process of easily identifying condensate build-up across steam heating coils over the length of an operational cycle and capturing, storing, trending, and displaying measurements of uniformity of heat distribution across the heating coils for evaluating system efficiency. These needs and others which will become apparent to one skilled in the art are provided by the present invention, which is summarized and described in detail below.